(Supported in part by NIH GMS 40198 to C.L. Rieder). In this study we are using laser microsurgery to determine when the RAD-9 checkpoint is turned off in vertebrate somatic cells. Studies in yeast reveal that mitosis is arrested when the chromosomes are damaged by irradiation. However, we know from work in our laboratory that vertebrate somatic cells do not respond the same way. Rather, in these cells mitosis proceeds normally when the chromosomes are severed by laser microsurgery after nuclear envelope breakdown. In the current study we are examining the effect of localized damage to the DNA and cytoplasm in controlling nuclear envelope breakdown. We find that damaging DNA in mid prophase cells returns them to interphase, but that the same experiment on late prophase cells does not inhibit entry into mitosis. We also found that irradiating the local regions of the cytoplasm with 532 nm light also caused mid but not late prophase cells to return to interphase. These results have been abstracted to the 1997 ASCB meeting and reveal that the RAD 9 checkpoint system is abrogated in vertebrate cells shortly prior to nuclear envelope breakdown. Cole, R.W., A. Khodjakov, C. Cogswell and C.L. Rieder. (1997) Progression of mid (but not late) prophase cells into prometaphase can be transiently reversed by irradiating the cytoplasm with intense green laser light. Mole. Biol. Cell 8: